Understanding the role of T cell immunity in the control and pathogenesis of dengue virus infection (DENV) is critical to development of a vaccine for prevention of DENV infection. The mechanistic basis for the initial lack of sufficient T cell control of DENV infection that leads to DHF, and the subsequent shift to protective T cell immunity in the host, is the primary focus of this project. We postulate that the interaction of DENV with dendritic cells (DC) leads to their hyper-activation and that of CDS and CD4 T cells, with excess production of vasoactive, proinflammatory cytokines and progression of DENV infection and disease. In Aim 1, we will determine the capacity of DC to activate and enhance the breadth and magnitude of the polyfunctional CDS and CD4 T cell recall responses to DENV. We expect that myeloid DC can enhance the breadth and magnitude of anti-DENV recall (memory) CDS and CD4 T cell polyfunctional responses that are essential for control of DENV infection for highly conserved and variable, serotype-specific, MHC class I and II DENV peptides using PBMC from a cross section of infected subjects from the US-Brazil CIEID cohort. In Aim 2, we will assess primary T cell responses to DENV proteins and their relationship to secondary T cell immune reactivity and "original antigenic sin". We believe that DC are potent inducers of primary CDS and CD4 T cell responses in DENV infection, and that this process is central to control and prevention of DHF in the host. Monocyte-derived DC, Langerhans cells and dermal DC will be compard for priming DENV-specific, polyfunctional CDS and CD4 T cell responses. In Aim 3, we will define the role of DC-activated, DENVspecific T cell cytokine expression and lineage and activation markers, and their control by regulatory T cells in the course of natural DENV infection. We postulate that DENV infection initially disrupts this process in persons who develop DHF by inducing hyper-activation of the DC and T cells, in part due to a decrease in the CD97-CD55 activation pathway by regulatory Tr1 cells. In summary, we propose to apply our state-ofthe- art immunologic approaches to assessing anti-DENV DC and T cell reactivity as part of the US-Brazil CIEID consortium. We believe that this study will reveal important correlates of immunity to DENV infection that will be important for treatment and vaccine initiatives.